APPROPRIATE USE OF BRIGHT LIGHT PROMOTES A DURABLE ADAPTATION TO NIGHT-SHIFTS AND ACCELERATES READJUSTMENT DURING RECOVERY AFTER A PERIOD OF NIGHT-SHIFTS

The present study examines the stability of bright Light circadian rea djustment during two consecutive dim light night-work periods and circ adian synchronization during the recovery after a night-shift period. A sample of 10 subjects was divided into 2 groups (control group: 6 su bjects; experimental group: 4 subjects). All subjects worked during 5 days, between 23:00 and 07:00 h and then went to sleep. Subjects recei ved 2500-3000 lux between 02:00 and 05:00 h during 5 days for the cont rol group and 3 days for the experimental group. During recovery after the night-shift periods, three cycles of bright light were administer ed at two different limes: 12:00-15:00 h for two of six subjects from the control group and 10:00-13:00 h for all subjects (4) of the experi mental group. By the fifth cycle of night-work the maximum of urinary aMT6s excretion that occurs at 05:00 h in the baseline condition was s hifted to 12:00 h for the control and experimental groups (delay in ho urs: 7 +/- 1.6 (control); 7 +/- 1 (experimental)). This result suggest s that three cycles of bright Light are sufficient to induce a signifi cant phase delay and that this delay remained stable when night-work p roceeded under dim light. The phase delay of the circadian aMT6s excre tion by exposure to bright light was accompanied by an improvement of the quality of day sleep and level of cognitive and psychomotor perfor mances for control and experimental groups. No significant difference was observed in the two groups for daytime sleep and nocturnal perform ance. The two bright light periods used during the three days of recov ery induced a complete synchronization in five of six subjects. One su bject showed a partial synchronization probably because he remained at the laboratory under dim light during the day and had few family and social contacts.